Best standard practice for commissioning a linear accelerator for clinical use typically requires a three dimensional (3D) water tank dosimetry scanner (3DS). A 2008 AAPM report1 “Accelerator Beam Data Commissioning Equipment and Procedures: Report of the TG-106 of the Therapy Physics Committee of the AAPM” (Indra Das—Chair) highlights the importance of the 3DS as well as a lack of easy to use systems currently available. The following excerpts provide guidance for the Performance Objectives of the 3DS.
From the above referenced TG-106 report (see Abstract): “For commissioning a linear accelerator for clinical use, medical physicists are faced with many challenges including the need for precision, a variety of testing methods, data validation, the lack of standards, and the time constraints. Since commissioning beam data is treated as a reference and ultimately used by treatment planning systems, it is vitally important that the collected data should be of the highest quality to avoid dosimetric and patient treatment errors that may subsequently lead to a poor radiation outcome. Beam commissioning data should be independent of the user and should be performed with appropriate knowledge and proper tools. To achieve this goal, Task Group 106 (TG-106) of the Therapy Physics Committee of the American Association of Physicists in Medicine (AAPM) was formed to review the practical aspects as well as the physics of linear accelerator commissioning.”
Again, from the TG-106 report (see Introduction): Beam data commissioning should be independent of users and scanning systems if it is performed with appropriate knowledge and proper tools. Data variation among collectors should be as minimal as possible (<1%). To achieve this goal, the TG-106 report was prepared to aid users in all aspects of accelerator beam data commissioning by describing specific set-up and measurement techniques, reviewing different types of radiation phantoms and detectors, discussing possible sources of error, and recommending procedures for acquiring specific photon and electron beam parameters.”
Also, the NEED, the PROBLEMS (issues), and the EFFORT of these measurements are defined with the following points that head a discussion on each. In particular, the time burden is emphasized in the third point:                “Need for commissioning data”        “Issues with beam commissioning measurements”        “Commissioning effort        “ . . . The amount of commissioning data requirements depends on the of the user's clinical need, including the treatment planning system (TPS), monitor unit programs, in-house data tables, and the like. To account for equipment setup, change in machine parameters, machine faults, etc, the typical time for photon beam scanning is 1.5 weeks. An additional week is needed for point data collections, 1-2 week for electrons and a week for verification. Typically, 1-2 weeks are needed in analysis and report writing. The typical time allotted for the commissioning process is 4-6 weeks. . . . ”        
Therefore, there is a need for an accurate scan measurement of relative dose in a water phantom. Furthermore, there is a need for the 3DS water tank size to permit at least a 40×40 cm2 field and a scanning depth of 40 cm1-II.A “Phantom material”. Furthermore, there is a need for the 3DS system to allow scanning in both cross- and in-plane (X and Y directions) and diagonal or star profiles1-Table1 and II.A. Quoting from the TG-106 report, (Section II. A Phantom Material) “Scanning in both dimensions provides convenience and avoids alignment problems associated with tank rotation.”
Further consider guidance from the TG-106 report (Section II.B Dimension of phantom):                “The size of water tank should be large enough to allow scanning of beam profiles up to the largest field size required (e.g., for photon beams, 40×40 cm2 with sufficient lateral buildup (5 cm) and over-scan distance. Some planning systems require larger lateral scans and diagonal profiles for the largest field size and at a depth of 40 cm for modeling. When considering the size of the scanning tank, the over scan and the beam divergence at 40 cm depth should be considered. A factor of 1.6 times the maximum field size should provide a safe limit. Simple calculation shows that at a maximum depth with consideration of over-scan and diagonal distance, a tank size of 75×75 cm2 is recommended. If the scanning software does not have the ability to perform diagonal scans, the table pedestal should be rotated to acquire the desired data. . . . The size of the tank still needs to be much larger than 75×75 cm2 to achieve the data with the same over-scan distance for diagonal profiles. In practical terms, however, very few commercial scanning systems are capable of scanning the full diagonal plus 5 cm over-scan at depths of >30 cm for 40×40 cm2 field at 100 cm SSD. Some compromise could be made by taking only half scans. Consequently, half scans will have to be collected for these maximum field sizes, which require an offset of the tank relative to the central axis. Before setting up for half scans, it is important to verify that the open beam show minimal asymmetry (<0.5%). . . . ”        
These guidelines are written by users of 3DS systems, keeping in mind the general concepts of 3DS systems that are commercially available. The guidelines, although published in 2008, are not new concepts since the 40 cm field sizes and TPS requirements have been around for decades. Thus, there has been a disconnect between a desirable scanning system to meet the performance needs of the application and what has actually been commercially available.
Water tank scanning dosimetry systems have been commercially available since the 1970s and probably earlier. Their designs incorporate orthogonal linear axes, the earlier units being a two axes system, one for depth and the other for horizontal “beam intensity profile” scans. To change a scan axis from beam in-plane to beam cross-plane, the operator would typically rotate the tank. Later as design sophistication came about, another horizontal axis was added (orthogonal to the other horizontal axis) making a three dimensional system, with the ability to scan to any location within the axes' scanning range. By the nature of the scan axes, these 2D and 3D systems used a “rectangular or orthogonal axis” geometry and were mounted in rectangular tanks that hold water. By way of example, Artronix Incorporated provided System 3302 three axis system in rectangular shape. It is of interest to note that a journal advertisement appeared in Medical Physics, 1976. This was a natural evolution to the radiation machines such as Co-60 units and linear accelerators (LINAC). The collimators ride on two axes, the in-plane and cross-plane, which produce square or rectangular radiation fields. Computer controls on linear axis drive systems were commonly available, making linear axes a natural selection. Scanning the beam to measure the radiation intensity distribution requires means to periodically measure the radiation “field” sensor, radiation detector, output at temporal or spatial increment positions as the sensor is moved through the water, and means to record these measurements for later analysis. The sensor will move perpendicular to the beam axis to measure the profile of beam intensity as a function of distance from the central axis of the beam. Such a movement will normally be parallel to the water surface when the beam is directed into the open top of the water tank, but could also be perpendicular to the water surface if the beam is directed through the sidewall of the water tank.
A measurement with the detector movement parallel to the beam axis would be a depth dose curve, i.e., the change in beam intensity as it transmits through the water and suffers beam divergence, otherwise known as “percent depth dose” (PDD). The measurement of the sensor is normally done in conjunction with a reference sensor that is stationary in the beam and positioned such that it does not interfere with the detector/sensor. Both sensors, radiation and reference, are measured simultaneously so that any change in beam intensity from the LINAC itself is normalized out by taking a ratio of the measurements.
Nearly all LINACs have a maximum field size of 40×40 cm; Varian2 LINACs have a primary collimator beam limiting geometry with rounded corners that result in a 50 cm maximum diagonal in a 40×40 cm field. Other manufacturers may have similar geometries. As discussed in the TG-106 report, this defines the tank geometry requirements if the scanner is to measure the beam and 5 cm outside of beam at both sides at a maximum depth of 40 cm. There is a need for scanning systems to perform these measurements. To overcome this in typical systems, the scanning system (and the tank) is shifted off center in order to measure the diagonal and 5 cm out of beam. For example, with the source to surface distance (SSD—water surface to LINAC target) at 90 cm, the 40×40 cm field at 40 cm depth extends to 47×47 cm. A 5 cm out of beam measurement extension requires an additional 10 cm, or a scan dimension of 57×57 cm. This exceeds the capabilities of most if not all commercially available scanners. The PTW3 MP3-M has approximate inner tank dimensions of 59.6 cm×59.4 cm and 50.6 cm depth. However, the scan dimensions are typically limited by the mechanical overhead of pillow blocks and stops that restrict the scan dimensions to approximately 54 cm×50 cm and 40.8 cm depth. The IBA Scanditronix Wellhofer4 RFA-300 has 49.5 cm×49.5 cm×49.5 cm scanning dimensions on the 3 linear orthogonal axes, again smaller than the desired 57×57 cm scan range when scanning all the geometries of a 40×40 cm field.
When the profile measurement nears the beam edge, there is a steep drop off in beam intensity as the sensor moves out of the beam. This beam edge, or “penumbra” region includes important information for the planning system and is used in commissioning the dose model of the treatment planning system (TPS) for the LINAC being commissioned. The shape of the penumbra region can be affected by the sensor geometry and if the sensor does not have scan direction symmetry, the relative penumbra shape may also be dependent upon the scan direction if the sensor is not re-oriented before scanning, i.e. does not have the same orientation for both scan directions. (See TG-106 §IV.A.4 Beam Profiles). Using a conventional three axis scanner, in order to keep the same detector orientation in profile scans that are orthogonal (ex: X and Y, cross-plane and in-plane, transverse and radial), the detector mount would be rotated 90 degrees. Some of the scanners have this provision with a detector mount that can be rotated, but this requires a trip into the LINAC room and runs the risk of disturbing the setup. A two dimensional scanner (one vertical, one horizontal) would require rotation of the scanner itself to make the orthogonal scan. It would keep the detector properly oriented but with the burden of a trip into the room and disturbance of the scanner setup.
The sensors are typically chosen by the medical physicists from an array of available sensors that may or may not be best suited for the measurement conditions, such as electrometer noise and signal (gain), field size, beam intensity from the LINAC, beam edge penumbra width, and beam type (electrons or X-rays). These issues are discussed in the TG-106 report and generally contribute to the problem of the beam scan measurement results not being unique to the beam but dependent on the operator and equipment.
Sensor size plays an important role on penumbra measurement, with larger dimensions in the scan direction contributing a larger error in the penumbra measurement. There are methods to correct for these “convolution” errors resulting from volume averaging of the sensor, as reported by JF. Dempsey5. However, this “de-convolution” correction method is complex and typically not available in the scanning systems. If corrected, as demonstrated by G Yan6, it would be done so after scanning, outside of the scanner system analysis software.
Therefore, there is a need for scan analysis, concurrent with the scanning system profile measurement, which provides a de-convolution of the chamber scan data that results in an accurate determination of the true beam profile shape and which provides the user the confidence to continue with the other beams before closing the LINAC measurements. A consistent data set is important for commissioning the TPS system, as stated in both TG-1061 and TG-537 reports. Consistency is best achieved in a contiguous measurements work flow that results when there is no need to repeat measurement in repeated setups.
The measurement session of the LINAC beam scanning can take many days as discussed in the TG-106 report. During these long scanning times, there are no assurances from the scanner system to indicate that the scanner system or the LINAC has not changed during scans in a way that would affect the measurement data. It is incumbent upon the operator to perform periodic quality assurance (QA) tests that would reveal such changes in the scanner system. This was the basic scope for TG-106 report, to provide insight to the operator who only occasionally performs the scanner measurements. There is a need in the scanner system to provide system QA tests which would reveal changes in the scanner operation that could cause or influence a change in scan measurements over the duration of the scanning sessions, both intersession i.e., between sessions separated by setup change, beam condition change (6 MV vs. 15 MV), day change, etc, and intrasession, i.e., within a session itself.
The measurement session of the LINAC beam scanning will consist of many setups and data structuring as discussed in the TG-106 report. During these many setup changes and tedious measurements, the operator may incorrectly identify data with particular setups. For example, unintentionally interchanging the labels on scan axes; or not changing the LINAC energy when the scan queue changed; or the collimator of the LINAC is rotated 90 degrees on a symmetric field without the user being aware. These types of setup errors are difficult to see after the sessions have ended and the data saved. The operator can open the data and examine the profiles, but there is generally not enough characteristic uniqueness to the data to easily identity an error, particularly if the operator is not very experienced, or even with experienced operators, when the error is a collimator rotation of 90 degrees. There is a need in the scanner system to provide setup QA tests that would reveal unique characteristics associated with the setup identifiers in the data that is to be saved.